Area Forecast Discussion
Issued by NWS Charleston, SC

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FXUS62 KCHS 230348

Area Forecast Discussion
National Weather Service Charleston SC
1048 PM EST Sun Jan 22 2017

A strong area of low pressure will develop over the Southeast
as it deepens near the southern Appalachians into tonight. This
allows for a strong cold front to push through the area
tonight. Weakening low pressure will pass by to the north on
Monday, before high pressure returns through mid week. Another
cold front will cross the area late in the week.


As of 1030 PM: Low pressure centered over NW GA continues to
deepen, latest observation down to 985 mb. Water vapor loop
showed the closed mid level low over middle GA, with dry air
sweeping east over the forecast area. Only a few SHRA and TSRA
remained off the coast of SC. I will update the forecast to
adjust PoP trends and limit the mention of TSRA to the SC
offshore. Otherwise, the current forecast appears in good shape.

As of 915 PM: The center of a 987 mb low was located over NW GA,
with a occluded front extending SE to the mouth of the Savannah
River. Over the past five minutes, winds at KCHS have gone from
calm to SE14G30kts. Buoys 41008 and 41004 have trended warmer
over the past two observations. Based on the latest observation,
I suspect that the warm front is running along the coast. I
will deep convection going off shore over the next couple of
hours, then all PoPs will transition to SHRAs. Otherwise, hourly
temps and dewpoints will be updated to adjust to latest LAV.

As of 8 PM, I will issue a update to cancel Tornado Watch 22.
Thunderstorms should continue generally east of I-95, these
storms may produce gusty winds and cloud to ground lightning.
Otherwise, I will make hourly temperature adjustments to reflect
latest guidance.

A quick update to remove the northern tier of counties from
Tornado Watch 22. The warm front has struggled to move north
across southern GA. Overlap of shear and instability has
remained pooled south of the warm front.

I will issue an update to add Tornado Watch 22, until 10 PM this
evening. A 440 PM, latest surface analysis indicated the center
of a 988 mb low was located over SW GA. Hi res models indicate
that the low center will rapidly lift closer to the I-20
corridor over the two hours. The associated warm front should
surge northward across the lower Savannah River Valley,
resulting in increase shear and instability. Based on SPC
mesoscale analysis trends and NAM12, a large region of 1.5 to
2.5 values of 0-1 km EHI will spread across the region early
this evening. This environment should support the ongoing
convection over middle and southern GA to spread east. These
storms may yield damaging winds and tornadoes.

Previous Discussion...
A surface low is rapidly strengthening across northeast AL.
This is resulting from a potent and abnormally deep closed low
currently moving over the lower MS valley. A cold front attached
to this low will be pushed through our forecast area during the
early part of tonight. Before the front moves through, severe
weather will occur across our area due to a very moist and
unstable atmosphere. We should note that, high temperatures
weren`t as warm as expected today. However, visible satellite
shows breaks in the clouds over southeast GA now, working their
way to the NNE. This is helping to maintain or warm up
temperatures a bit as we head towards daybreak. There remains
plenty of dynamics and impressive shear this evening. A strong
low level jet, mid level dry air, and the overall large scale
forcing for ascent with impressive dynamics and upper divergence
are in place. This combined with negative LI`s, SBCAPES, and
impressive shear values for anytime of year, let alone in
January. SHERBS3 values of 1-1.5 and a Sig Tor approaching 3
indicate the potential for strong, long-tracked tornadoes within
discrete supercells and bowing segments. SPC continues to have
much of our GA counties under a HIGH risk of Severe Weather,
with Moderate risk elsewhere. This is unheard of for January!

A dry slot will develop behind the cold front as it sweeps
offshore late this evening and after midnight. Rain will quickly
decrease, but we`re keeping POPs in the forecast until daybreak
to account for the potential of some late night showers moving
back into our NW tier as the upper low approaches.

While winds have increased ahead of the front, they are not
high enough to need a wind advisory. Plus, the strongest winds
will be associated with the line of convection itself.

More likely is winds across Lake Moultrie will reach at least
20 kt sustained and/or 25 kt gusts tonight, so Lake Wind
Advisory is in effect.


The general trend through the first half of next work week will be
improving conditions as ridging gradually builds in aloft. Surface
low pressure will fill as it moves across the Carolinas and off Cape
Hatteras Monday. Our proximity to the disturbance will allow for
some modest destabilization, but lack of significant moisture will
preclude anything other than a slight chance for some light
rain during the day Monday.

High pressure will build into the area from the south as the low
departs Monday night and drier air will allow for abundant sunshine
and a rain-free Tuesday and Wednesday. Temps will remain above
normal through the period.

Lake Winds: Winds will become gusty around and over Lake Moultrie on
Monday. A lake wind advisory remains in effect through early Monday
evening, through confidence is low in the wind event.


A cold front will approach Wednesday night and then cross the area
Thursday. Moisture looks to be fairly limited so current PoP scheme
only features 20-30% chances. High pressure will return in its wake
and persist through much of the weekend. Temperatures above normal
Thursday will finally moderate closer to normal behind the cold


A Line of thunderstorms will pass over the region through 2Z.
These storms should bring gusty SW winds with TEMPO MVFR to IFR
vis and ceilings. Once the line of storms pass, gusty SW winds
should continue into the late night hours. Winds may weaken
during the pre dawn hours as the low center tracks toward the
coast. However, in the wake of the low winds will increase with
gusts in the low 20s, veering from the west. Another round of
weak showers is expected to pass over KCHS during the early
daylight hours.

Extended Aviation Outlook: Periods of flight restrictions possible
Monday as low pressure departs the area. VFR thereafter.


Tonight: SCA and Gale Warnings are up for various portions of
the waters. These occur with large isallobaric pressure falls
followed by strong height falls aloft.

Monday through Friday: Hazardous marine conditions will be
ongoing at the start of the period as a strong low pressure
system remains nearby. At least Small Craft Advisories will be
in effect for all waters including the Charleston Harbor. There
will be a chance for gale force gusts, especially in the outer
Georgia waters. Will continue to assess trends. The low will
lift northeast away from the area on Tuesday and winds/seas will
begin to subside. High pressure will build in its wake and
prevail through mid week with no marine issues expected. A cold
front will cross the waters Thursday with another round of small
craft advisories possible.


SC...Lake Wind Advisory until 7 PM EST Monday for SCZ045.
MARINE...Small Craft Advisory until 7 AM EST Tuesday for AMZ352-354.
     Gale Warning until 1 AM EST Monday for AMZ350-374.
     Small Craft Advisory until 1 AM EST Tuesday for AMZ330.


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